1. Field of the Invention
The present invention relates to a liquid crystal display device, driving control circuit and driving method used in the same display device, and more particularly to the liquid crystal display device having an LED (Light Emitting Diode) backlight and suitably used when a moving image is displayed, the driving control circuit and the driving method used in the same display device.
2. Description of the Related Art
To display a television image, a CRT (Cathode Ray Tube) has been conventionally used, however, in recent years, liquid crystal display devices are widely used. In each of the liquid crystal display devices, a liquid crystal display panel is a non-luminous panel and, therefore, a backlight is placed on a rear of the liquid crystal panel and images are displayed by changing transmittance of light emitted from the backlight according to an input video signal. However, the liquid crystal panel as the related art has a problem. That is, when black is displayed on the liquid crystal panel, if a backlight is ordinarily turned ON, leakage of light from a displaying surface of the liquid crystal panel occurs, which causes contrast to be degraded.
In addition, a CRT display panel is self-luminous and, therefore, a dynamic range of luminance can be widened by changing peak luminance according to an input video signal, whereas the liquid crystal display panel in the liquid crystal display device is non-luminous and, as a result, widening of a dynamic range of luminance is difficult. Another problem is that, when moving images are displayed by the liquid crystal display device, since a response of a liquid crystal to an applied voltage requires time and since a holding-type driving operation is performed in which a current frame is held until a video signal corresponding to a succeeding frame is supplied, a trail-leaving (afterimage) phenomenon occurs. Thus, an improved liquid crystal display device that tries to solve these problems is proposed.
Related art technology of this type is disclosed in, for example, in Japanese Patent Application Laid-open No. 2005-258404 (Abstract, FIGS. 13 and 30). The disclosed liquid crystal display device, as shown in FIG. 15, includes an external light sensor 1, a controller 2, a display data changing circuit 3, a backlight light amount controlling circuit 4, a liquid crystal display section 5, a backlight 6, and an optical sensor 7. In the disclosed liquid crystal display device, the controller 2 controls, based on a signal “pe” output from the optical sensor 7 to detect emission of light from the backlight 6, on an image signal “vf” input to be used for displaying on the liquid crystal display section 5, and on a signal “pg” output from the external light sensor 1 to detect external environmental light, a process of changing displaying data for each color (red (R), green (G), and blue (B)) in the liquid crystal display section 5 and an amount of light, for each color (R, G, B), emitted from the backlight 6.
In the disclosed case, as shown in FIG. 16, the backlight 6 is turned ON in every frame period with time width corresponding to converting indices from 100 to 255 of luminance of an illumination light source that change in inverse proportion to optical transmittance of liquid crystals. As a result, contrast of a displayed image and a dynamic range of luminance are improved. In addition, by flashing of the backlight 6, a trail-leaving phenomenon of moving images on a displayed screen is reduced.
However, the above liquid crystal display device as the related art has the following problems. That is, in the related art liquid crystal display device in FIG. 15, as shown in FIG. 17A, when a gray level of a video signal to be input is comparatively low, a response of a liquid crystal is slow which causes lighting timing of the backlight 6 to be delayed, whereas, when a gray level of a video signal to be input is comparatively high, a response of the liquid crystal is rapid, which causes rapid lighting timing of the backlight 6. Thus, lighting timing of the backlight 6 differs greatly depending on the gray level of a video signal to be input and, therefore, when a moving picture is displayed, a degree of a trail-leaving phenomenon differs depending on the gray level of an input video signal, causing degradation of quality of images.